Angsuman Roy

High-Speed Buffer

​When I was working on my Master's Thesis, one problem I ran into was measuring the noise spectrum from my sigma-delta modulators. The problem was that they could not drive the 50 ohm input of the spectrum analyzer in our lab. My solution was to create a low-distortion high-speed buffer. Low distortion is important so as not to corrupt the SNDR measurements. High-speed is important to faithfully preserve the noise-shaping of the sigma-delta modulator. From my audio hobby I noticed that an "audio buffer" chip, the LME49610 had a bandwidth of 200 MHz. This is overkill for audio applications but could be useful in solving my measurement problem. With a low distortion of 0.005%, adding the buffer would be essentially transparent as my ADC had a resolution of less than 10 bits.

To preserve the high bandwidth into a 50-ohm load, I decided to parallel three LME49610 chips. I added series resistors to the output of each chip to control for variations in output resistance. Next, I added a series terminating resistor such that the equivalent output resistance was equal to 50 ohm. I tested the contraption on a network analyzer and it was completely flat to 180 MHz. I'll have to repeat those measurements and post them in the future.